As much as I'd like to get at this issue with a high-tech solution, I'm not sure it would work, plus it would take some effort to implement.

One example: Small sign above a column of doorbells "Don't knock, please ring bell" Each bell would be labeled, starting from the top, "Family/Friends," "Other Neighbors," "Deliveries," "Everyone else." The first three would ring through and the last one would do something else. A pre-recorded message telling them to leave, turn on a hose to wash down every inch of area in front of the door or perhaps a shock. Maybe the message on first press and then the shock and water with the second press (on any of the bells).